Board Game Arena

So yesterday after winning a game of Piraten Kapern I reached 6th all-time in Elo (I'm kind of embarrassed that I'm doing so well at a game that is 99% luck while I'm extremely bad at the games requiring skill ). Before I played my next game, one of the players above me lost, promoting me to 5th. I never got the "Top 5 Elo" achievement, presumably because the eligibility for achievements is only determined at the end of a game to the players in that game?

Is this done intentionally, or is it an oversight?

It is as you say, the rank only gets checked for you when you finish a game / your ELO changes. I guess anything else would be too ressource heavy / complex.

So you need to finish one game and stay in the reached tier with it, to get the trophy.